Voluntary control of postural equilibrium patterns

Alignment of the Sternum and Sacrum as a Marker of Sitting Body Posture in Children

An analysis of literature on the methods of assuming a sitting position and the results of our own research indicated the need to search for biomechanical parameters and existing relationships that would enable a description of sitting body posture. The purpose of this paper is to analyze the relationship between the alignment of the body of sternum and sacrum and the changes in the thoracic and lumbar spine curvatures in children. The study involved 113 subjects aged 9-13 years. A planned simultaneous measurement of the angle parameters of the alignment of the body of sternum and sacrum relative to the body's sagittal axis and the angle parameters of the thoracic and lumbar spine curvatures was performed during a single examination session. The proposed markers of alignment in the corrected sitting body posture are characterized by homogeneous results. A high measurement repeatability was observed when determining the corrected body posture in the study setting. It was noted that changes in the alignment of the body of sternum and sacrum resulted in changes in the thoracic kyphosis and lumbar lordosis angle values, which may be an important component of clinical observations of sitting body posture in children. Implementing the body of sternum alignment angle of about 64° relative to the body's sagittal axis in clinical practice as one of the objectives of postural education may be the target solution for sitting body posture correction in children.

Adaptation of balancing behaviour during continuous perturbations of stance. Supra-postural visual tasks and platform translation frequency modulate adaptation rate

When humans are administered continuous and predictable perturbations of stance, an adaptation period precedes the steady state of balancing behaviour. Little information is available on the modulation of adaptation by vision and perturbation frequency. Moreover, performance of supra-postural tasks may modulate adaptation in as yet unidentified ways. Our purpose was to identify differences in adaptation associated to distinct visual tasks and perturbation frequencies. Twenty non-disabled adult volunteers stood on a platform translating 10 cm in antero-posterior (AP) direction at low (LF, 0.18 Hz) and high frequency (HF, 0.56 Hz) with eyes open (EO) and closed (EC). Additional conditions were reading a text fixed to platform (EO-TP) and reading a text stationary on ground (EO-TG). Peak-to-peak (PP) displacement amplitude and AP position of head and pelvis markers were computed for each of 27 continuous perturbation cycles. The time constant and extent of head and pelvis adaptation and the cross-correlation coefficients between head and pelvis were compared across visual conditions and frequencies. Head and pelvis mean positions in space varied little across conditions and perturbation cycles but the mean head PP displacements changed over time. On average, at LF, the PP displacement of the head and pelvis increased progressively. Adaptation was rapid or ineffective with EO, but slower with EO-TG, EO-TP, EC. At HF, the head PP displacement amplitude decreased progressively with fast adaptation rates, while the pelvis adaptation was not apparent. The results show that visual tasks can modulate the adaptation rate, highlight the effect of the perturbation frequency on adaptation and provide evidence of priority assigned to pelvis stabilization over visual tasks at HF. The effects of perturbation frequency and optic flow and their interaction with other sensory inputs and cognitive tasks on the adaptation strategies should be investigated in impaired individuals and considered in the design of rehabilitation protocols.

Stabilization of body balance with Light Touch following a mechanical perturbation: Adaption of sway and disruption of right posterior parietal cortex by cTBS

Light touch with an earth-fixed reference point improves balance during quite standing. In our current study, we implemented a paradigm to assess the effects of disrupting the right posterior parietal cortex on dynamic stabilization of body sway with and without Light Touch after a graded, unpredictable mechanical perturbation. We hypothesized that the benefit of Light Touch would be amplified in the more dynamic context of an external perturbation, reducing body sway and muscle activations before, at and after a perturbation. Furthermore, we expected sway stabilization would be impaired following disruption of the right Posterior Parietal Cortex as a result of increased postural stiffness. Thirteen young adults stood blindfolded in Tandem-Romberg stance on a force plate and were required either to keep light fingertip contact to an earth-fixed reference point or to stand without fingertip contact. During every trial, a robotic arm pushed a participant's right shoulder in medio-lateral direction. The testing consisted of 4 blocks before TMS stimulation and 8 blocks after, which alternated between Light Touch and No Touch conditions. In summary, we found a strong effect of Light Touch, which resulted in improved stability following a perturbation. Light Touch decreased the immediate sway response, steady state sway following re-stabilization, as well as muscle activity of the Tibialis Anterior. Furthermore, we saw gradual decrease of muscle activity over time, which indicates an adaptive process following exposure to repetitive trials of perturbations. We were not able to confirm our hypothesis that disruption of the rPPC leads to increased postural stiffness. However, after disruption of the rPPC, muscle activity of the Tibialis Anterior is decreased more compared to sham. We conclude that rPPC disruption enhanced the intra-session adaptation to the disturbing effects of the perturbation.

Postural Instability in Parkinson's Disease: A Review

Parkinson’s disease (PD) is a heterogeneous progressive neurodegenerative disorder, which typically affects older adults; it is predicted that by 2030 about 3% of the world population above 65 years of age is likely to be affected. At present, the diagnosis of PD is clinical, subjective, nonspecific, and often inadequate. There is a need to quantify the PD factors for an objective disease assessment. Among the various factors, postural instability (PI) is unresponsive to the existing treatment strategies resulting in morbidity. In this work, we review the physiology and pathophysiology of postural balance that is essential to treat PI among PD patients. Specifically, we discuss some of the reported factors for an early PI diagnosis, including age, nervous system lesions, genetic mutations, abnormal proprioception, impaired reflexes, and altered biomechanics. Though the contributing factors to PI have been identified, how their quantification to grade PI severity in a patient can help in treatment is not fully understood. By contextualizing the contributing factors, we aim to assist the future research efforts that underpin posturographical and histopathological studies to measure PI in PD. Once the pathology of PI is established, effective diagnostic tools and treatment strategies could be developed to curtail patient falls.

Emergence of Lissajous Patterns as a Function of a Perturbation Frequency in Postural Responses to the Short Sinusoidal Translations of Varying Frequencies

The existence of in-phase and anti-phase postural responses to sinusoidal perturbations to the base of support is well known. In this study, we investigate if such coordinated postural responses exist at 'near-sway' perturbations where the perturbation amplitudes are kept within the range of normal sway lengths in healthy adults (n=10). The postural responses are analyzed via bursts of anterior-posterior (AP) 2.5 mm horizontal sinusoidal oscillations of the base of support at sequentially varying frequencies (0.25, 0.375, 0.5, 0.625, 0.75, 1 and 1.25 Hz). The parametric plots of the perturbation signal (platform position) and the response profiles (AP Center of Pressure [APCoP]) show the emergence of elliptical Lissajous patterns as the perturbation frequency is increased from 0.25 Hz to 1.25 Hz. The presence of such characteristic pattern shows the 'lock-in' behavior of APCoP with perturbation signal. These elliptical patterns become more apparent at the center frequencies (0.375 to 0.75 Hz). At the higher frequencies (1 and 1.25 Hz), the Lissajous patterns do exist but are dominated by low-frequency drift. The area and orientation of Lissajous patterns and the phase shifts between perturbation and APCoP show a strong nonlinear decreasing trend with increasing perturbation frequency for both, young (n=5) as well as mature (n=5) adults within the study group. This may suggest that such characteristic, frequency-locked, phased shifted response of healthy posture control could be a fundamental property of a healthy posture control's response to 'near-sway' sinusoidal translations in AP direction.

Regulation of dynamic postural control to attend manual steadiness constraints

In daily living activities, performance of spatially accurate manual movements in upright stance depends on postural stability. In the present investigation, we aimed to evaluate the effect of the required manual steadiness (task constraint) on the regulation of dynamic postural control. A single group of young participants ( n = 20) were evaluated in the performance of a dual posturo-manual task of balancing on a platform oscillating in sinusoidal translations at 0.4-Hz (low) or 1-Hz (high) frequencies while stabilizing a cylinder on a handheld tray. Manual task constraint was manipulated by comparing the conditions of keeping the cylinder stationary on its flat or round side, corresponding to low and high manual task constraints, respectively. Results showed that in the low oscillation frequency the high manual task constraint led to lower oscillation amplitudes of the head, center of mass, and tray, in addition to higher relative phase values between ankle/hip-shoulder oscillatory rotations and between center of mass/center of pressure-feet oscillations as compared with values observed in the low manual task constraint. Further analyses showed that the high manual task constraint also affected variables related to both postural (increased amplitudes of center of pressure oscillation) and manual (increased amplitude of shoulder rotations) task components in the high oscillation frequency. These results suggest that control of a dynamic posturo-manual task is modulated in distinct parameters to attend the required manual steadiness in a complex and flexible way.

NEW & NOTEWORTHY We evaluated dynamic postural control on a platform oscillating in sinusoidal translations at different frequencies while performing a manual task with low or high steadiness constraints. Results showed that high manual task constraint led to modulation of metric and coordination variables associated with greater postural stability. Our findings suggest that motor control is regulated in an integrative mode at the posturo-manual task level, with reciprocal interplay between the postural and manual components.

Devices and tasks involved in the objective assessment of standing dynamic balancing - A systematic literature review

BACKGROUND

Static balancing assessment is often complemented with dynamic balancing tasks. Numerous dynamic balancing assessment methods have been developed in recent decades with their corresponding balancing devices and tasks.

OBJECTIVE

The aim of this systematic literature review is to identify and categorize existing objective methods of standing dynamic balancing ability assessment with an emphasis on the balancing devices and tasks being used.

DATA SOURCES

Three major scientific literature databases (Science Direct, Web of Science, PLoS ONE) and additional sources were used.

STUDY SELECTION

Studies had to use a dynamic balancing device and a task described in detail. Evaluation had to be based on objectively measureable parameters. Functional tests without instrumentation evaluated exclusively by a clinician were excluded. A total of 63 articles were included.

DATA EXTRACTION

The data extracted during full-text assessment were: author and date; the balancing device with the balancing task and the measured parameters; the health conditions, size, age and sex of participant groups; and follow-up measurements.

DATA SYNTHESIS

A variety of dynamic balancing assessment devices were identified and categorized as 1) Solid ground, 2) Balance board, 3) Rotating platform, 4) Horizontal translational platform, 5) Treadmill, 6) Computerized Dynamic Posturography, and 7) Other devices. The group discrimination ability of the methods was explored and the conclusions of the studies were briefly summarized.

LIMITATIONS

Due to the wide scope of this search, it provides an overview of balancing devices and do not represent the state-of-the-art of any single method.

CONCLUSIONS

The identified dynamic balancing assessment methods are offered as a catalogue of candidate methods to complement static assessments used in studies involving postural control.

Functional contributors to poor movement and balance control in patients with low back pain: A descriptive analysis

BACKGROUND

Automatic and voluntary body position control is essential for postural stability; however, little is known about individual factors that impair the sensorimotor system associated with low back pain (LBP).

OBJECTIVE

To evaluate automatic and voluntary motor control impairments causing postural instability in patients with LBP.

METHODS

Motor control impairments associated with poor movement and balance control were analyzed prospectively in 32 patients with LBP. Numeric Rating Scale (NRS) for pain assessment, Oswestry Disability Index (ODI) for disability measurement, and computerized dynamic posturography (CDP) for analysis of postural responses were used to measure outcomes of all patients. Computerized dynamic posturography tests including Sensory organization test (SOT), limits of stability test (movement velocity, directional control, endpoint, and maximum excursion), rhythmic weight shift (rhythmic movement speed and directional control), and adaptation test (toes-up and toes-down tests) were performed and the results compared with NeuroCom normative data.

RESULTS

The mean age of the patients was 40.50 ± 12.28 years. Lower equilibrium scores were observed in SOT (p < 0.05). There was a significant increase in reaction time and decrease in movement velocity, directional control, and endpoint excursion (p < 0.05). Speed of rhythmic movement along the anteroposterior direction decreased, while speed increased along the lateral direction (p < 0.05). Poor directional control was recorded in the anteroposterior direction (p < 0.05). Toes-down test showed an increased COG sway in patients compared with that in the controls (p < 0.05).

CONCLUSIONS

LBP causes poor voluntary control of body positioning, a reduction in movement control, delays in movement initiation, and a difficulty to adapt to sudden surface changes.

Upper body balance control strategy during continuous 3D postural perturbation in young adults

We explored how changes in vision and perturbation frequency impacted upright postural control in healthy adults exposed to continuous multiaxial support-surface perturbation. Ten subjects were asked to maintain equilibrium in standing stance with eyes open (EO) and eyes closed (EC) during sinusoidal 3D rotations at 0.25 (L) and 0.50 Hz (H). We measured upper-body kinematics--head, trunk, and pelvis--and analyzed differences in horizontal displacements and roll, pitch, and yaw sways. The presence of vision significantly decreased upper-body displacements in the horizontal plane, especially at the head level, while in EC the head was the most unstable segment. H trials produced a greater segment stabilization compared to L ones in EO and EC. Analysis of sways showed that in EO participants stabilized their posture by reducing the variability of trunk angles; in H trials a sway decrease for the examined segments was observed in the yaw plane and, for the pelvis only, in the pitch plane. Our results suggest that, during continuous multiaxial perturbations, visual information induced: (i) in L condition, a continuous reconfiguration of multi-body-segments orientation to follow the perturbation; (ii) in H condition, a compensation for the ongoing perturbation. These findings were not confirmed in EC where the same strategy--that is, the use of the pelvis as a reference frame for the body balance was adopted both in L and H.

Effect of loading parameters on motor performance during a dynamic weight-shift task

Controlling weight shift (WS) is essential to performing motions safely and smoothly during daily and athletic activities. This study investigated the impact of loading parameters on the motor performance and difficulty level of a dynamic WS task performed while standing. Twelve healthy young adults (21.2 ± 0.9 years, 53.5 ± 7.4 kg) were asked to match the target and their weight loads using visual feedback displayed on a computer monitor. Motor performance was estimated by assessment of loading accuracy and pace of motor skill acquisition, measured as a proxy of difficulty level, was estimated by assessment of learning rate. As predicted, both loading accuracy values decreased with increasing target frequency. Notably, the interaction of loading size and frequency had a significant effect on loading accuracy, which was increasingly impaired as the weight load increased at frequencies greater than 0.5 Hz. Moreover, the pace of motor skill acquisition in a dynamic WS task while standing was affected by the weight-load speed independently of the weight-load size. These results indicate that loading accuracy is affected by both the weight-load size and frequency and that 0.5 Hz is a critical frequency at which the difficulty level increases during dynamic WS tasks while standing in healthy youths. These findings suggest that the adequacy of the initial settings used regarding loading size and frequency is an important consideration in rehabilitative and athletic training aimed at evaluating and improving WS while standing.

Transition of COM-COP relative phase in a dynamic balance task

The purpose of this study was to investigate whether the coordination between center of mass (COM) and center of pressure (COP) could be a candidate collective variable of a dynamical system that captures the organization of the multi-segmental whole body postural control system. We examined the transition of the COM-COP coordination pattern in a moving platform balance control paradigm. 10 young healthy adults stood on a moving surface of support that within a trial was sinusoidally translated in the anterior-posterior direction continuously scaling up and then down its frequency within the range from 0Hz to 3.0Hz. The COP was derived from a single force platform mounted on the moving surface of support. 4 angular joint motions (ankle, knee, hip, and neck) were measured by a 3D motion analysis system that also allowed COM to be derived. The COM-COP coordination changed from in-phase/anti-phase to anti-phase/in-phase at a certain frequency of the support surface, showed hysteresis as a function of the direction of frequency change and higher variability at the transition region. Conversely, the transition of the ankle-hip coordination consistently occurred at 0.3Hz across subjects with little between or within subject variability as a function of transition frequency and before the COM-COP transition. The findings provide evidence that: (1) the transition of the COM-COP coordination pattern is that of a non-equilibrium phase transition with critical fluctuations and hysteresis; and (2) that COM-COP coupling is a candidate collective variable of the multi-segmental whole body postural control system acting on a redundant postural task.

The effect of ankle and knee immobilization on postural control during standing

BACKGROUND

This study examined the effects of ankle and knee joint immobilization on postural control in healthy young adults while standing.

METHODS

The 24 participants included in this study participated in postural standing tests under four different constraint conditions: free joints, ankle immobilization only, knee immobilization only, and ankle-knee immobilization. Tests were performed using a commercial balancing equipment (Biodex(TM), Inc., NY, USA) and software.

RESULTS

The overall limit-of-stability score and duration to completion of task were obtained at 75% limit-of-stability (moderate level of difficulty). The overall limit-of-stability score of free joints (34.5±9.1) were significantly different with the ankle immobilization only (26.0±11.6), and ankle-knee immobilization (26.4±7.4) conditions. The test duration increased in the following order: free joints (57.8±10.9), knee immobilization only (62.5±14.0), ankle immobilization only (68.0±14.9), and ankle-knee immobilization (69.4±17.7). The duration of the free joint condition greatly decreased than ankle immobilization only and ankle-knee immobilization conditions.

CONCLUSIONS

The ankle and knee joints have sufficient range of motion to prevent falls related to decrease postural stability due to insufficient range of motion or to improve postural stability.

Effect of balance training on postural balance control and risk of fall in children with diplegic cerebral palsy

PURPOSE

The purpose of this study was to evaluate the effects of balance training on postural control and fall risk in children with diplegic cerebral palsy.

METHODS

Thirty spastic diplegic cerebral palsied children (10-12 years) were included in this study. Children were randomly assigned into two equal-sized groups: control and study groups. Participants in both groups received a traditional physical therapy exercise program. The study group additionally received balance training on the Biodex balance system. Treatment was provided 30 min/d, 3 d/week for 3 successive months. To evaluate the limit of stability and fall risk, participated children received baseline and post-treatment assessments using the Biodex balance system. Overall directional control, total time to complete the test, overall stability index of the fall risk test and total score of the pediatric balance scale were measured.

RESULTS

Children in both groups showed significant improvements in the mean values of all measured variables post-treatment (p < 0.05). The results also showed significantly better improvement in the measured parameters for the study group, as compared to the control group (p < 0.05).

CONCLUSION

Balance training on Biodex system is a useful tool that can be used in improving postural balance control in children with diplegic cerebral palsy.

Postural coordination patterns as a function of rhythmical dynamics of the surface of support

This study investigated the organization of postural coordination patterns as a function of the rhythmical dynamics of the surface of support. We examined how the number and nature of the dynamical degrees of freedom in the movement coordination patterns changed as a function of the amplitude and frequency of support surface motion. Young adult subjects stood on a moving platform that was translated sinusoidally in anterior-posterior (AP) direction with the task goal to maintain upright bipedal postural balance. A force platform measured the kinetics at the surface of support and a 3D motion analysis system recorded torso and joint kinematics. Principal components analysis (PCA) identified four components overall, but increasing the average velocity of the support surface reduced the modal number of components of the postural coordination pattern from three to two. The analysis of joint motion loadings on the components revealed that organizational properties of the postural pattern also changed as a function of platform dynamics. PC1 (61.6-73.2 %) was accounted for by ankle, knee, and hip motion at the lowest velocity conditions, but as the velocity increased, ankle and hip variance dominated. In PC2 (24.2-20.2 %), the contribution of knee motion significantly increased while that of ankle motion decreased. In PC3 (9.7-5.1 %) neck motion contributed significantly at the highest velocity condition. Collectively, the findings show that the amplitude and frequency of the motion of the surface of support maps redundantly though preferentially to a small set of postural coordination patterns. The higher platform average velocities led to a reduction in the number of dynamical degrees of freedom of the coordination mode and different weightings of joint motion contributions to each component.

Comparison of reliability, validity, and responsiveness of the mini-BESTest and Berg Balance Scale in patients with balance disorders

BACKGROUND

Recently, a new tool for assessing dynamic balance impairments has been presented: the 14-item Mini-BESTest.

OBJECTIVE

The aim of this study was to compare the psychometric performance of the Mini-BESTest and the Berg Balance Scale (BBS).

DESIGN

A prospective, single-group, observational design was used in the study.

METHODS

Ninety-three participants (mean age=66.2 years, SD=13.2; 53 women, 40 men) with balance deficits were recruited. Interrater (3 raters) and test-retest (1-3 days) reliability were calculated using intraclass correlation coefficients (ICCs). Responsiveness and minimal important change were assessed (after 10 sessions of physical therapy) using both distribution-based and anchor-based methods (external criterion: the 15-point Global Rating of Change [GRC] scale).

RESULTS

At baseline, neither floor effects nor ceiling effects were found in either the Mini-BESTest or the BBS. After treatment, the maximum score was found in 12 participants (12.9%) with BBS and in 2 participants (2.1%) with Mini-BESTest. Test-retest reliability for total scores was significantly higher for the Mini-BESTest (ICC=.96) than for the BBS (ICC=.92), whereas interrater reliability was similar (ICC=.98 versus .97, respectively). The standard error of measurement (SEM) was 1.26 and the minimum detectable change at the 95% confidence level (MDC(95)) was 3.5 points for Mini-BESTest, whereas the SEM was 2.18 and the MDC(95) was 6.2 points for the BBS. In receiver operating characteristic curves, the area under the curve was 0.92 for the Mini-BESTest and 0.91 for the BBS. The best minimal important change (MIC) was 4 points for the Mini-BESTest and 7 points for the BBS. After treatment, 38 participants evaluated with the Mini-BESTest and only 23 participants evaluated with the BBS (out of the 40 participants who had a GRC score of ≥ 3.5) showed a score change equal to or greater than the MIC values.

LIMITATIONS

The consecutive sampling method drawn from a single rehabilitation facility and the intrinsic weakness of the GRC for calculating MIC values were limitations of the study.

CONCLUSIONS

The 2 scales behave similarly, but the Mini-BESTest appears to have a lower ceiling effect, slightly higher reliability levels, and greater accuracy in classifying individual patients who show significant improvement in balance function.

Modifications of muscle synergies and spinal maps due to absence of visual feedback in patients with unilateral vestibular disease

The present study aimed at describing the modifications of muscle synergies and spinal activity due to the absence of visual feedback, in patients affected by unilateral vestibular disease. Patients were tested both during unperturbed quite stance and walking while the activity of 7 bilateral muscles, from the leg to the trunk, were recorded for the estimation of muscle synergies and spinal activity. Results showed that during locomotion the absence of visual feedback did not significantly modify either the principal roles underlying muscle activity (i.e., synergies) or the spinal bursts. Conversely, during the upright stance, the absence of visual feedback involved a significant coupling of ankle dorsi- and plantar-flexor muscle groups with a consequent shift of the motoneuronal (MN) activity toward most caudal segments. Results revealed that the muscle synergies are able to document an increased activity of sensory-motor afferences leading a more intense role of the forward based mechanism underlying balance control in vestibular patients.

Equilíbrio estático da criança com baixa visão por meio de parâmetros estabilométricos

INTRODUÇÃO: A visão é considerada um dos mais importantes meios de captação das informações do ambiente, sendo fundamental para o processo de desenvolvimento da criança. OBJETIVOS: Cientes da participação da visão na manutenção do equilíbrio e no desenvolvimento biopsicossocial do ser humano e considerando os fatores decorrentes da perda ou limitação visual, esta pesquisa objetivou analisar o equilíbrio em postura ortostática de crianças com baixa visão, de faixa etária de 8 a 11 anos, de ambos os sexos (grupo experimental), comparando com o equilíbrio de crianças normais de ambos os sexos e mesma faixa etária (grupo controle), por meio de parâmetros estabilométricos. MATERIAIS E MÉTODOS: Para a obtenção das oscilações posturais (deslocamento radial e velocidade de deslocamento) foi utilizada uma plataforma estabilométrica. RESULTADOS: Considerando-se os índices (média e desvio padrão) de deslocamento radial apresentados pelos grupos experimental e controle, observou-se que não houve diferença significante na condição olhos fechados (p > 0,05). Já na condição olhos abertos, houve maior deslocamento radial do grupo experimental em relação ao grupo controle (p < 0,05). Analisando-se os índices (média e desvio padrão) de velocidade de deslocamento entre os grupos controle e experimental, observou-se que em ambas as condições (olhos abertos e olhos fechados) o grupo controle apresentou maior velocidade de deslocamento quando comparado ao grupo experimental (p < 0,01). CONCLUSÃO: A partir dos resultados apresentados, concluímos que a baixa visão parece influenciar negativamente a estabilidade em postura ortostática, bem como a velocidade de ajuste postural, prejudicando o equilíbrio corporal.

Differences in rapid initiation and termination of voluntary postural sway associated with ageing and falls-risk

The authors examined differences between young adults (n = 25) and healthy older adults (n = 48) in reaction time and the relations between center of pressure (COP) and center of mass (COM) motions during rapid initiation and termination of voluntary postural sway. Older adults were divided into low and high falls-risk groups based on Physiological Profile Assessment scores of sensorimotor function. Low falls-risk older adults had slower reaction times during anteroposterior sway initiation and decreased COP-COM separation during anteroposterior and medialateral sway initiation and anteroposterior continuous voluntary sway compared with young adults. High falls-risk older adults had slower initiation and termination reaction times in all response directions and decreased COP-COM separation during sway initiation and continuous voluntary sway in the anteroposterior and medialateral directions compared with young adults. Compared with low falls-risk older adults, high falls-risk older adults had slower initiation and termination reaction times in all response directions and decreased COP-COM separation during medialateral continuous voluntary sway. Reaction time and COP-COM measures significantly predicted group status in discriminant models with sensitivities and specificities of 72-100%. Overall, these findings highlight important associations of age-related declines in sensorimotor function related to an increased risk of falling with slower postural reaction time and reduced postural stability.

Modeling of postural stability borders during heel-toe rocking

To maintain balance during movements such as bending and reaching, the CNS must generate muscle forces to counteract destabilizing torques produced by gravitational (position-dependent) and inertial (acceleration-dependent) forces. This may create a trade-off between the attainable frequency and amplitude of movements. We used experiments and mathematical modeling to examine this relationship during the task of heel-toe rocking. During the experiments, participants (n=15) rocked about the ankles in the sagittal plane with maximum attainable amplitude at a frequency of 0.33 Hz or 0.66 Hz. As the frequency doubled, the maximum anterior position of the whole-body centre-of-gravity (COG) with respect to the ankle decreased by 11% of foot length (from 11.9 cm (S.D. 1.6) to 9.2 cm (S.D. 1.2); p<0.001), the minimum anterior position of the COG increased by 8% of foot length (from 1.6 cm to 3.5 cm in front on the ankle; p<0.0005), and the ankle stiffness increased from 787 Nm/rad (S.D. 156) to 1625 Nm/rad (S.D. 339). However, there was no difference between conditions in the maximum anterior position of the COP (p=0.51), the minimum anterior position of the COP (p=0.23), or the peak ankle torque (p=0.39). An inverted pendulum model driven by a rotational spring predicted the measured ankle stiffness to within 0.9% (S.D. 6.8), and the maximum anterior COG position to within 1.2% (S.D. 4.0). These results indicate that COG amplitude decreases with increasing rocking frequency, due to (a) invariability in peak ankle torque and (b) the need to allocate torque between gravitational and inertial components, the latter of which scales with the square of frequency.

Effect of changing visual condition and frequency of horizontal oscillations on postural balance of standing healthy subjects

The goal of this study was to describe the movement pattern of the body-segment rotations of healthy subjects in the horizontal plane while they were standing on a supporting platform that imposed steady sinusoidal horizontal rotations under three visual conditions: (a) eyes closed with no instructions (EC-NI), (b) eyes open with instructions to gaze at a stationary black dot located at eye level on a wall surface about four meters in front of them (EO-WI), and (c) eyes closed with instructions to imagine looking at the same target (EC-WI). The selected input signal was a sinusoid with an amplitude of +/-45 deg at different frequencies equal to 0.25, 0.50 and 0.75 Hz, which were referred to as L, M and H. Bipedal balance measurements were taken in 10 adult subjects (mean age 30+/-9 years; three men and seven women). Subjects' kinematics were analyzed with an optoelectronic system. Under the three visual conditions, the movements of the pelvis, the trunk, and the head decreased and were inversely dependent on platform frequency; specifically, both the head and the trunk decreased their gain rotation of about 1.8-2.9 times from L to H, while the pelvis decreased its by about 1.3 times. However, the arm oscillations showed a gain and phase tendency opposite to that of the other body segments, with the gain rotation having increased of about 1.8-3.7 times from L to H. Comparing the three visual conditions, the finding suggests that the subjects were able to stabilize their head as a reference frame to maintain postural balance in a similar way under the EC-WI and EO-WI conditions. Instead, in the EC-NI trials, the subjects compensated less, in particular at the hip, the external perturbation producing higher absolute body rotations and lower relative body rotations. In fact, the head rotation was about four and three times the one showed in EC-WI and EO-WI, while for the trunk and the pelvis it was always equal to two and 1.5 times the correspondent rotation observed under the WI conditions. These results provide a quantitative assessment of compensatory balance reactions in healthy subjects to periodical horizontal perturbations.

The control of equilibrium in Parkinson's disease patients: delayed adaptation of balancing strategy to shifts in sensory set during a dynamic task

Processing of sensory information, timing operations and set-shifting can be affected in Parkinson's disease (PD) patients. We investigated their capacity and swiftness to pass from a kinaesthetic- to a vision-dependent behaviour during dynamic balancing on a continuously moving support base. Nineteen on-phase PD patients and 13 age-matched normal subjects stood on a platform continuously translating in the antero-posterior direction at 0.2 Hz. Body segment oscillations were identified by a stereophotogrammetric device and electromyogram (EMG) was recorded from tibialis anterior and soleus. Under constant visual conditions, both patients and normal subjects roughly stabilised head and trunk in space with eyes open (EO) but followed the platform displacement with eyes closed (EC). Amplitude and variability of the periodic EMG bursts were smaller with EO than EC. Constant visual-condition trials were intermingled with trials in which subjects opened (EC-EO) or closed (EO-EC) the eyes in response to an acoustic signal. Both patients and normal subjects changed kinematics and EMG patterns to those appropriate for the new visual condition. However, PD patients were slower in changing their behaviour under the EC-EO condition. These findings show abnormal temporal features in balancing strategy adaptation when shifting from kinaesthetic to visual reference in PD. The delay in the implementation of the vision-dependent behaviour was unexpected, given the advantage vision is supposed to confer to motor performance in PD. This condition may play a major role in the instability of patients performing dynamic postural tasks under changing sensory conditions.

Time to reconfigure balancing behaviour in man: changing visual condition while riding a continuously moving platform

While balancing on a continuously antero-posterior (A-P) translating platform (10 cm, 0.5 Hz), the head normally oscillates with the platform without vision but is stabilized in space with vision. We estimated the time to shift from one to the other balancing behaviour when visual condition changed at some stage during the balancing trials. Ten subjects performed randomly 50 balancing trials (each lasting 18 s): 10 trials with eyes open (EO), 10 with eyes closed (EC), 15 in which participants started with EO and closed their eyes (condition EO-->EC) in response to an acoustic signal delivered during the trial, and 15 starting with EC and closing their eyes (EC-->EO) in response to the same signal. No other specific instruction was given. Displacements of malleolus, hip and head, and EMG from leg and axial muscles were recorded. Indexes of amplitude of A-P head and hip oscillation and of amplitude of EMG activity were computed. All variables were larger with EC than EO. On changing visual condition during the trial, the pattern of head and hip movement and of muscle activity turned into that appropriate for the new visual condition in a time-interval ranging from about 1 to 2.5 s. For each subject, the mean latency of the change in the balancing behaviour was assessed by statistical methods. On average, the latencies of kinematics and EMG changes proved to be longer for the EO-->EC condition than vice versa. Further, the latencies of the changes were also measured across all EO-->EC and EC-->EO individual trials. These values were clustered around particular epochs of the first few oscillation cycles following the shift in visual condition. The results show that subjects can rapidly adapt their balancing behaviour to the new visual condition. However, they appear to refrain from releasing the new behaviour were this unfit, and unfastened it at appropriate time in the next platform translation cycle. These findings reveal the temporal and spatial features of the automatic release of the new balancing strategy in response to a shift in the ongoing sensory set, and emphasize the swiftness in the change in balancing behaviour when subjects pass from a non-visual to a visual reference frame.

Two Kinematic Synergies in Voluntary Whole-Body Movements During Standing

We used a particular computational approach, the uncontrolled manifold hypothesis, to investigate joint angle covariation patterns during whole-body actions performed by standing persons. We hypothesized that two kinematic synergies accounted for the leg/trunk joint covariation across cycles during a rhythmic whole-body motion to stabilize two performance variables, the trunk orientation in the external space and the horizontal position of the center of mass (COM). Subjects stood on a force plate and performed whole-body rhythmic movements for 45 s under visual feedback on one of the four variables, the position of the center of pressure or the angle in one of the three joints (ankle, knee, or hip). The Fitts-like paradigm was used with two target amplitudes and six indices of difficulty (ID) for each of the four variables. This was done to explore the robustness of kinematic postural synergies. A speed-accuracy trade-off was observed in all feedback conditions such that the movement time scaled with ID and the scaling differed between the two movement amplitudes. Principal-component (PC) analysis showed the existence of a single PC in the joint space that accounted for over 95% of the joint angle variance. Analysis within the uncontrolled manifold hypothesis has shown that data distributions in the joint angle space were compatible with stabilization of both trunk orientation and COM location. We conclude that trunk orientation and the COM location are stabilized by co-varied changes of the major joint angles during whole-body movements. Despite the strong effects of movement amplitude and ID on performance, the structure of the joint variance showed only minor dependence on these task parameters. The two kinematic synergies (co-varied changes in the joint angles that stabilized the COM location and trunk orientation) have proven to be robust over a variety of tasks.

Aging effects on postural responses to self-imposed balance perturbations

The present study investigated how young and older individuals organize their posture in response to self-induced balance perturbations evoked by oscillatory single limb movements. Eleven old (70.1+/-4.3 years) and nine young (20.1+/-2.4 years) participants performed repeatedly for 5s hip flexion/extension movements using full range of motion and maximum velocity. Two-dimensional joint kinematics (sampling rate: 60Hz), center of pressure (CoP) and EMG activity of tibialis anterior (TA), medial gastrocnemius (MGAS) rectus femoris (RF) and, semitendinosus (ST) in the stance limb were recorded and analysed. Cross-correlation function (CCF) analysis was used to identify the degree of coupling between the swinging limb (SL), center of gravity (CoG) and CoP motions. Old adults significantly limited SL, CoG and CoP range of anterior/posterior (A/P) motion in response to the forceful leg swinging. In the stance limb, significantly lower levels of ankle muscle activity resulted in reduced hip and knee joint excursions and increased ankle instability. By contrast, young performers produced sufficient ankle muscle activity to stabilize the foot to the ground while progressively increasing joint range of motion from the ankle to the hip. Center of pressure and SL movements were strongly correlated in an anti-phase relationship in both age groups. In older adults, however, the relationship between CoG-SL and CoG-CoP movements was neither strong nor synchronous, reflecting a weaker coupling and lack of coordination between component movements. It is concluded that insufficient ankle muscle activity, central integration deficits and increased anxiety to postural threat are important factors implicated for the weaker postural synergies and freezing of degrees of freedom seen in the elderly during performance of single limb oscillations.

Influence of a portable audio-biofeedback device on structural properties of postural sway

BACKGROUND

Good balance depends on accurate and adequate information from the senses. One way to substitute missing sensory information for balance is with biofeedback technology. We previously reported that audio-biofeedback (ABF) has beneficial effects in subjects with profound vestibular loss, since it significantly reduces body sway in quiet standing tasks.

METHODS

In this paper, we present the effects of a portable prototype of an ABF system on healthy subjects' upright stance postural stability, in conditions of limited and unreliable sensory information. Stabilogram diffusion analysis, combined with traditional center of pressure analysis and surface electromyography, were applied to the analysis of quiet standing tasks on a Temper foam surface with eyes closed.

RESULTS

These analyses provided new evidence that ABF may be used to treat postural instability. In fact, the results of the stabilogram diffusion analysis suggest that ABF increased the amount of feedback control exerted by the brain for maintaining balance. The resulting increase in postural stability was not at the expense of leg muscular activity, which remained almost unchanged.

CONCLUSION

Examination of the SDA and the EMG activity supported the hypothesis that ABF does not induce an increased stiffness (and hence more co-activation) in leg muscles, but rather helps the brain to actively change to a more feedback-based control activity over standing posture.